1. Field of the Invention
The present invention relates to a power transmitting mechanism including a silent chain trained around a driving sprocket mounted to a drive shaft and a driven sprocket mounted to a driven shaft.
2. Description of the Related Art
A power transmitting mechanism formed by a combination of a silent chain and sprockets is widely used in a transfer case of a four-wheel drive vehicle, a power transmitting apparatus disposed between an engine and a transmission, and a camshaft drive unit of an engine. In a power transmitting mechanism used, for example, in the transfer case, an output shaft of a transmission and a second output shaft parallel to the output shaft are each equipped with one sprocket. The second output shaft is connected to a front axle or a rear axle. A silent chain is trained around these sprockets for transmitting power from a drive shaft consisting of the output shaft of the transmission to a driven shaft formed by the second output shaft.
Noise generated in such power transmitting mechanism mainly includes a collision sound (meshing impact sound) emitted when the silent chain comes into meshing engagement with teeth of the sprockets, and a resonance sound caused by a periodic collision sound. Various technologies have been proposed to decrease the noise level, however, there is a strong demand for further reduction of the noise level.
Japanese Patent Laid-open Publication No. HEI-7-167232 discloses a combined silent-chain and sprocket system used for driving a camshaft of an engine with the aim of reducing resonance sound. The disclosed system comprises two driving sprockets mounted side by side on a single drive shaft, a driven sprocket mounted on each of two driven shafts a first silent chain trained around one driving sprocket on the drive shaft and the driven sprocket on one of the driven shafts, and a second silent chain trained around the other driving sprocket on the drive shaft and the driven sprocket on the other driven shaft.
The driving sprockets on the drive shaft have respective teeth offset or phased with respect to one another. The first and second silent chains come into meshing engagement with the corresponding driving sprockets on the drive shaft with a time difference therebetween. With this arrangement, the disclosed system can exhibit a certain effect in reducing the resonance sound.
The disclosed system seeks to reduce resonance sound by making respective tooth pitches of the driving sprockets different from each other and randomizing respective pitches of the silent chains.
However, since the sprockets and silent chains of the conventional system (power transmitting mechanism) are both made irregular in pitch, more than two differently configured link plate sets must be provided for making up a single extended silent chain. Considering that the silent chain is tailored for each individual item of equipment such as a transfer case or an engine, this requirement would result in considerable increases in the cost of dies due for press-forming of the link plate, the cost of inventory of the link plates, and the cost of assembling the silent chain.
Another problem associated with the conventional power transmitting mechanism is that since the silent chains and sprockets both have irregular pitch, the meshing timing between the silent chains and the mating sprockets tends to vary widely.
For instance, when irregularities in the sprockets and silent chains cancel out each other, an expected resonance sound reducing effect cannot be achieved. Conversely when irregularities in the sprockets and silent chains are added together, meshing impact sound becomes large due to an excessively large pitch difference existing between the sprockets and the chains. This may result in rapid wear of meshing surfaces (flanks) of the silent chains and sprockets.
In the case where the conventional power transmitting mechanism is used for driving a camshaft of a vehicle engine, the above-mentioned excessive large pitch difference caused by the added chain and sprocket irregularities greatly influences the valve timing, causing a drop in engine output.
With the foregoing drawbacks of the prior art in view, it is an object of the present invention to provide a power transmitting mechanism which is capable of reducing resonance sound without causing an increase in the level of collision sound and undue wear of a silent chain and sprockets.
Another object of the present invention is to provide a power transmitting mechanism which is economical but can sufficiently decrease the resonance sound.
According to the present invention, there is provided a power transmitting mechanism comprising first and second driving sprockets mounted to a single drive shaft, the first and second driving sprockets having a plurality of spaced teeth, the teeth of the first driving sprocket and the teeth of the second driving sprocket being of substantially identical pitch and being offset from each other by one-half pitch. The power transmitting mechanism also comprises first and second driven sprockets mounted to a single driven shaft, the first and second driven sprockets being aligned with the first and second driving sprockets, respectively, a first silent chain trained around the first driving sprocket and the first driven sprocket, and a second silent chain trained around the second driving sprocket and the second driven sprocket.
The teeth of the first driving sprocket and the teeth of the second driving sprockets have a random pitch which varies in a random pattern along the circumference of the first and second driving sprockets, and the first and second silent chains have a single pitch.
The power transmitting mechanism is particularly suitable for use in a transfer case of a four-wheel drive vehicle or a power train between an engine and a transmission, wherein power is transmitted between a single drive shaft and a single driven shaft, or in an camshaft drive mechanism of a double overhead camshaft (DOHC) engine, wherein power is transmitted between a single drive shaft and two driven shafts. In the later application, the power-transmitting operation is achieved by using either a single silent chain trained around three sprockets mounted to the respective shafts, or alternatively two silent chains one of which is trained around a driving sprocket on the drive shaft and a driven sprocket on one driven shaft, and the other silent chain being trained around a driving sprocket and a driven sprocket on the other drive shaft.
In the power transmitting mechanism, when the drive shaft is rotating, power is transmitted from the drive shaft to the driven shaft via the silent chains running around the driving and driven sprockets. Each of the silent chains generates an impact sound as it enters the corresponding sprocket from the free span of the chain. According to the present invention, the drive force is evenly shared by the two silent chains disposed side by side, the peak value of the impact sound emitted from each individual silent chain is relatively low. In addition, since the teeth of the first driving sprocket are offset from the teeth of the second driving sprocket by one-half pitch, the impact sound emitted from the first silent chain and the impact sound emitted to the other silent chain are cancel out each other. Thus, the total noise level of the power transmitting mechanism is considerably reduced.
The tooth pitch of the driving sprockets is a random pitch which varies in a random pattern along the circumference of the sprockets, while the silent chains both have a single pitch. Accordingly, the periodicity of the meshing impact sound caused by meshing engagement between each silent chain and the corresponding driving sprocket is controlled solely by variations of the tooth pitch of the driving sprocket. The tooth pitch variations of the driving sprockets are neither canceled out by, nor added with, variations of the pitch of the silent chains. It is, therefore, possible to reduce the resonance sound effectively, and prevent undue increase in the meshing impact sound and accelerated wear of the chain and sprocket contact surfaces which may occur when the pitch difference between the silent chains and the driving sprockets becomes large.
In one preferred form, some teeth of each of the first and second driving sprockets are thinned or thickened along one flank thereof by a thickness which ranges from 0.5 to 2.0% of a pitch of other teeth of each driving sprocket. It is preferable that the number of the thinned or thickened teeth is in the range of 10 to 50% of a total number of the teeth of each the driving sprocket.
In order to maintain the random pitch, it is preferable that the first and second driving sprocket are part of a single driving sprocket assembly of unitary construction, and the driving sprocket assembly is formed from a sintered metal. The one-piece driving sprocket assembly formed of a sintered metal can be manufactured at a low cost as compared to a conventional one-piece sprocket manufactured though cutting and rolling processes, and can readily maintain the offset between the first and second sprocket teeth over a longed period of use.
It is also preferable that the teeth of the first driven sprocket and the teeth of the second driven sprocket are of substantially identical pitch and are offset from each other by one-half pitch. The respective teeth of the first and second driven sprockets may have a single pitch which is uniform throughout the circumference of the driven sprockets, or a random pitch which varies. in a random pattern along the circumference of the driven sprockets. In the case where the driven sprockets have a random tooth pitch, the same noise suppressing effect as attained by the random tooth pitch of the driving sprockets can be also attained by the random pitch of the driven sprockets.